Distilled water harmful? I owuld not worry.
Yes I am aware of the discussion on possible harm of distilled water to aluminium parts. The issue has to do with `very' pure distilled water may react rapidly ionically with bare metal surface. This does not bother me much as we do not usually have sure high purity in our distilled water and as long as the coolant is pre-mixed before being poured in, we are safe. I would opt for MB coolant ot be safe though I realize some member have good experience with aftermarket ones.